I think you are mistaking speaking with precision for an English accent.

The only characters that have English Accents are Obi-Wan (considering that Sir Alec Guinness was British, Ewan MacGregor should probably endevor to sound a bit like him. Right?

"But here's another interesting note -- all of the Jedi (with only two exceptions) have Asian sounding names. Yoda. Obi-wan Kenobi. Qui-gon Jinn. Perhaps George Lucas was aiming for a certain Samurai connection in that -- or maybe, like most of his half-planned "nuances" it was in fact laziness or accident that led to the pattern."

In actual fact, they are fully intended to sound "Asian." This is because Lucas was greatly influenced by the films of Akira Kurosawa, particularly The Seven Samurai. The Force corresponds roughly to the Eastern concept of 'chi,' an energy that can supposedly be harnessed by certain very skilled people and used to perform superhuman feats.

And Vader even looks and fights like a Samurai warrior. He strikes with powerful diagonal slashes of his lightsaber. His armor and mask look similar to samurai armor.